In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements (lithographic printing plate precursors) useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the imageable element is considered as positive-working. Conversely, if the non-imaged regions are removed, the imageable element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The lithographic printing plate precursors used for the preparation of lithographic printing plates have been designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads of more usually, infrared laser diodes that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to image the elements.
These imaging techniques often require the use of water or a developer (neutral to alkaline pH) as a processing solution to remove exposed (positive-working) or non-exposed (negative-working) regions of the imaged layer(s). In general, the processing solution is specifically designed for the specific radiation-sensitive chemistry in the imaged precursor and to provide processing as cleanly as possible.
During processing of imaged lithographic printing plate precursors, some processing solutions cause corrosion of aluminum-containing substrates and the industry has been investigating means for reducing or preventing corrosion. For example, some processing solutions contain various corrosion inhibitors and in other instances, the aluminum-containing substrates are pretreated to reduce corrosion.
Some commercial dual-layer positive-working lithographic printing plate precursors are imagewise exposed using infrared radiation and then processed with a processing solution containing a silicate as a corrosion inhibitor as described for example in U.S. Patent Application Publications 2004/0063036 (Takamiya) and 2009/0162783 (Levanon et al.). However, it is well known that silicate-containing processing solutions become “dirty” with sludge after continued use. In addition, when silicate dries in the processing solution tank or other parts of the equipment, carbon dioxide from the air neutralizes the silicate to form crusted silicon dioxide that is difficult to remove during maintenance operations.
Other commercial plate makers design positive-working lithographic printing plate precursors for processing with processing solutions containing non-reducing sugars as corrosion inhibitors as noted also in U.S. Patent Application Publication 2004/0063036 (noted above). However, these materials are generally less effective than silicates and are effective only with aluminum-containing substrate that have been pretreated for example using a post-anodic phosphate/fluoride treatment solution that itself is prone to form insoluble deposits on rollers and equipment surfaces used in the post-anodic treatment process, and thus requires frequent laborious cleaning.
It would be desirable to process imagewise exposed positive-working lithographic printing plate precursors using processing solutions that are substantially free of silicate, without requiring that the precursors be made using a dirty post-anodic treatment process.